Holographic interference lithography (HIL) involves the intersection of two or more laser beams to form periodic sub-wavelength nanostructures of photosensitive materials, such as photopolymers and glasses. In some configurations, a single laser beam may be used in conjunction with a diffracting photomask grating, wherein diffracted laser beams interfere to generate periodic patterns in the near field. The minimum periodicity that may be achieved using this method is about half of the laser wavelength. Periodic sub-wavelength nanostructures of materials other than photopolymers may be formed by incorporating chemical etching and/or material deposition techniques.
It is known that when a material is exposed to an intense laser beam close to its melting threshold, the surface of the material may spontaneously develop light-induced periodic surface structures (LIPSSs). The most accepted theories interpret LIPSSs as being the result of inhomogeneous energy distribution caused by the interference between the incident laser beam and a scattered surface wave and/or reflected/refracted light. The spacing of LIPSSs is determined by the laser wavelength, incident angle, polarization, and, if the surface wave is inside of the material, the refractive indices of the substrates. Recently, using intense femtosecond pulsed lasers, periodic sub-wavelength nanostructures, such as cracks and/or refractive index modifications, have been obtained inside or on the surface of transparent bulk materials. These features have been shown to scale with laser wavelength. However, due to their disruptive nature, these features may only be produced using amplified femtosecond pulses with a pulse energy that is at least a few hundred nanojoules or an intensity that is at least a few hundred terawatts per square centimeter.
LCVD involves the dissociation of precursors and the subsequent deposition of desired materials on a substrate induced by a laser beam, either pyrolytically or photolytically, as opposed to by direct substrate heating. Using LCVD, a variety of materials may be deposited via the selection of the appropriate precursors. LCVD provides advantages over conventional thermal chemical vapor deposition (CVD) in terms of reduced processing temperature and improved feature size spatial resolution.
Direct-writing (DW-LCVD) involves the initiation of the localized decomposition of precursor molecules either photo-thermally, photo-chemically, or by a combination of both using a laser beam. Advantageously, it may be used to pattern materials that are incompatible with conventional photolithography, thereby reducing processing steps to minimize cross-contamination and/or cost, reduce processing temperature on heat-sensitive substrates, and fabricate functional structures three-dimensionally. A variety of precursors have been developed for DW-LCVD, to deposit materials ranging from metals, to semiconductors, to dielectrics, rendering DW-LCVD a versatile tool for rapid prototyping. The feature size obtained by DW-LCVD is limited by diffraction to about half of the laser wavelength. Higher spatial resolution may be achieved by exciting precursor molecules either at near field or using a shorter wavelength. However, the former suffers from low throughput of light, while the latter requires a short-wavelength light source and associated optics, which are not readily available. Another method of achieving higher spatial resolution is by utilizing a nonlinear process during deposition, such as the diffusion and nucleation of radicals on the surface, exploiting the temperature dependence of the thermal reaction rate, and the multi-photon absorption-induced decomposition of precursors. For example, the DW-LCVD of chromium nanowires using chromium hexacarbonyls on a variety of substrates has been demonstrated by employing a 100-femtosecond 400-nm pulsed laser. Line widths as small as 100 nm (corresponding to λ/4, where λ is the vacuum wavelength of the laser beam) have been achieved as a result of the combination of the multi-photon absorption-induced decomposition of precursors and the tight focusing of incident light (NA=0.9), which makes the further reduction of feature size very difficult.
DW-LCVD is, by its nature, a serial process that is slow as compared to the parallel process of conventional photolithography. In order to form a two-dimensional feature, such as a plurality of lines or a patch, multiple scans with appropriate offsets between the scans are required. In addition, the morphology of the deposition in conventional DW-LCVD follows, either linearly or nonlinearly, the energy distribution at the laser focus, which is a Gaussian function or the like. This results in depositions with smooth profiles and limited morphological variations, such as lines, dots, or a combination of both.
Thus, what is still needed in the art is a DW-LCVD technique that is capable of generating a feature size of λ/5 or smaller, for example, processing multiple features simultaneously, and controlling the morphology of the deposited features, among other advantages.